I. Field of the Invention
The present invention relates generally to systems for transmitting, receiving and recognizing digital signals representative of fax calls. More particularly, the present invention relates to systems and methods for quickly and reliably detecting fax calls. Even more particularly, the present invention relates to networks that operate in multiple modes in order to support both fax and voice transmissions, and to systems and methods for reliably switching such networks between their fax and voice modes.
II. Description of the Related Art
For a standard Group 3 fax communication system, the timing and synchronization between fax machines conform to the requirements specified in "ITU-T Recommendation T.30: Procedures for Document Facsimile Transmission in the General Switched Telephone Network," hereinafter referred to as "the T.30 Fax Protocol," and incorporated herein by reference. The T.30 Fax Protocol uses a number of modulation techniques for transmission of message data. In particular, parameter negotiation and handshaking between fax machines is accomplished using the modulation technique specified in "CCITT Recommendation V.21: 300 bps Duplex Modem Standard for Use in the General Switched Telephone Network," hereinafter referred to as "V.21" or "the V.21 Standard" and also incorporated herein by reference.
In a fax system that conforms to the T.30 Fax Protocol, messages conforming to the V.21 Standard (hereinafter referred to as "V.21 messages") are sent between a calling fax machine and a called fax machine in establishing a call. A calling fax machine initiates a call by dialing the called fax machine and sending a calling tone (referred to as a CNG tone). The called fax machine detects the incoming call and sends a called station identification tone (referred to as a CED tone) back to the calling fax machine. The called fax machine then sends its digital identification signal to the calling fax machine to inform the calling fax machine of its capabilities. Upon detection of this signal, the calling fax machine sends a digital command signal to inform the called fax machine of the capabilities that the calling fax machine plans to use. Other messages in addition to the above-described initiation signals also take place between fax machines during a call. Such messages include, for example, training signals, information messages and termination messages.
In a fax system that conforms to the T.30 Fax Protocol, each V.21 message sent must include a preamble (referred to as "the V.21 preamble"), which consists of a series of 32 flags (referred to as "the V.21 flags.") The preamble is used to inform the receiver that a message is to be sent, and allows the receiver to synchronize correctly so that the actual message is properly received.
Digital communications systems and networks often support both voice and fax transmissions. These systems process these two forms of data differently and require switching of resources depending on whether voice or fax service is being supported. This switching must be performed quickly and reliably so that the system does not lose relevant fax messages or send fax messages through the system while it is operating in voice mode.
When a dual-mode network is operating in voice mode, the V.21 flags at the beginning of a fax call represent a reliable method of detecting a fax call, because the flag pattern is easily distinguishable from typical voice signals. However, in order to detect the flags reliably, it is necessary to listen for a threshold number of repetitions of the flags before declaring a fax call. Since each flag lasts for 266.7 milliseconds, waiting for these repetitions to occur can require substantial time. However, having to wait too long before switching from voice mode to fax mode can cause the system to lose some of the fax messages. Moreover, not switching from voice mode to fax mode fast enough can cause a significant portion of the initial fax tones to pass through the voice path of the system, thereby corrupting the fax tones or triggering a premature negotiation between the fax machines. In order to prevent such premature negotiation when a network is operating in voice mode, the network must detect the first V.21 message and switch from voice mode to fax mode before the receiving fax machine receives the message.
In order to shorten the time required to detect the V.21 flags, one approach would be to simply lessen that number of flag repetitions that are used as the threshold for declaring that a fax call is in progress. However, this approach can cause a degradation in switching reliability because, if the detector is forced to make a switching decision without a sufficient amount of data, the detector may falsely detect a fax call and switch from voice mode to fax mode. Such a mistake would not be acceptable to users having a conversation, since their conversation would abruptly terminate.
These problems and deficiencies are recognized and solved by the present invention in the manner described below.